json-validator/include/jvalidate/detail/out.h

#pragma once

#include <type_traits>
#include <utility>
#include <variant>

namespace jvalidate::detail {
constexpr struct discard_out_t {
} discard_out;

/**
 * @brief An optional out-parameter to a function, similar to a function
 * that takes `T* out_param = nullptr`. Unfortunately, std::optional does not
 * support storing references - so if we want the syntactic sugar of that, we
 * need a custom class.
 *
 * In addition to acting like an optional value - we have the special behavior
 * that we wanted - namely that "if there is a contained value provided by the
 * caller, we will update that reference", and "if there is no value, then
 * assigning a value will have no effect" as if we performed the idiomatic:
 * @code
 * if (out_param) {
 *   *out_param = ...;
 * }
 * @endcode
 *
 * @tparam T The type being returned
 */
template <typename T>
  requires(std::is_same_v<T, std::decay_t<T>>)
class out {
private:
  T * ref_ = nullptr;

public:
  // Construct an empty out parameter, that has no side-effects
  out() = default;
  // Explicitly construct an empty out parameter, that has no side-effects
  out(discard_out_t) {}
  // Construct an out parameter pointing to the given concrete value.
  out(T & ref) : ref_(&ref) {}

  /**
   * @breif On rare occasions, we still need to perform checks
   * that an out-param holds a value.
   */
  explicit operator bool() const { return ref_; }

  /**
   * @brief Update the value of this out parameter, if it holds a value. By
   * convention, we assume that this function will only be called once, but
   * there is no requirement for that.
   *
   * @tparam U Any type that can be used to construct the held type T
   *
   * @param val The new value being passed up to the caller
   *
   * @returns Nothing - this object does not behave like a normal object where
   * you can do things like `if ((A = B).foo())` - since this object represents
   * exclusively a way to pass an optional value back to the caller without
   * returning a tuple.
   */
  template <typename U>
    requires std::is_constructible_v<T, U>
  void operator=(U && val) {
    if (ref_) {
      *ref_ = std::forward<U>(val);
    }
  }
};

/**
 * @brief A non-optional out-parameter to a function, similar to a function that
 * takes a `T& out_param` argument. Unlike the standard form, this type allows
 * passing an rvalue (temporary) or an lvalue (persistant) element, and will
 * properly handle the assigment and updating of the object as appropriate.
 *
 * @tparam T The type being returned
 */
template <typename T>
  requires(std::is_same_v<T, std::decay_t<T>>)
class inout {
private:
  std::variant<T, T *> ref_;

public:
  // Constructs an inout parameter from an rvalue type - whose modification will
  // not effect the calling scope.
  inout(T && value) : ref_(std::move(value)) {}
  // Constructs an inout parameter from an lvalue type - whose modification will
  // propogate upwards to the caller.
  inout(T & ref) : ref_(&ref) {}

  /**
   * @brief Convert this object back into its underlying type for use.
   *
   * @returns A reference to the contained value/reference, to avoid the cost
   * of performing a copy-operation if the contained object is non-trivial.
   */
  operator T const &() const {
    struct {
      T const & operator()(T const & in) const { return in; }
      T const & operator()(T * in) const { return *in; }
    } visitor;
    return std::visit(visitor, ref_);
  }

  /**
   * @brief Update the value of this out parameter. Depending on the variation
   * contained in this type, this will either propogate up to the caller's level
   * or will update future uses of this object.
   *
   * @tparam U Any type that can be used to construct the held type T
   *
   * @param val The new value being set
   *
   * @returns The updated value
   */
  template <typename U>
    requires std::is_constructible_v<T, U>
  T const & operator=(U && val) {
    struct {
      U && val;
      T const & operator()(T & in) const { return in = std::forward<U>(val); }
      T const & operator()(T * in) const { return *in = std::forward<U>(val); }
    } visitor{std::forward<U>(val)};
    return std::visit(visitor, ref_);
  }
};
}